The human brain is not a finished organ at birth -- in fact, another 10 or 12
years are needed before even a general development is completed. Structural maturation
of individual brain regions and their connecting pathways is required for the
successful development of cognitive, motor, and sensory functions. This maturation
eventually provides for a smooth flow of neural impulses throughout the brain,
which allows for information to be integrated across the many spatially segregated
brain regions involved in these functions. The speed of neural transmission is
an important factor, and this depends not only on the junctions between nerve
cells (synapses), but also on the structural properties of the connecting fibers
(axons). Critical axon structural properties include their diameters and the thickness
of the special insulation (myelin) around many fibers. Large groups of myelinated
axons, which connect various regions in the brain, appear visibly as "white matter".
Axons of the major pathways in the human brain, such as those of the corpus callosum
(which connects the two halves of the brain) or the corticospinal tract (which
connects the brain to the spinal cord and the rest of the body), continue to develop
throughout childhood and adolescence. Postmortem studies suggest that axon diameters
and myelin sheaths undergo conspicuous growth during the first two years of life,
but may not be fully mature before adolescence or even late adulthood. The scarcity
of human brain specimens for postmortem analysis has made it difficult to draw
definite conclusions about the timetable of myelinization during childhood and
adolescence.

Our understanding of the propagation of nerve impulses represents an interesting
convergence of physics and biology. The nerve impulse is a rapid propagating wave
(approximately 1 millisecond in duration) of depolarization followed by repolarization.
In the language of physics, the neuron axon behaves as an electrical transmission
line with a transverse time-variant and voltage-dependent negative conductance
element in parallel with a high capacitance. In fact, the equations describing
the propagation of neuron action potentials derive from the classical equations
for wave propagation along electrical transmission lines developed by Maxwell
and Kelvin. As expected from these equations, the cross- sectional diameter of
an axon is an important determinant of impulse propagation velocity: the larger
the diameter, the greater the velocity of propagation. The myelin sheath that
surrounds certain types of axons is a periodically interrupted electrical insulation,
and on physical grounds it can be demonstrated that the effect of this type of
insulation, considering the known electrical properties of the axon, is a substantial
increase in pulse propagation velocity over that of a bare axon of the same diameter.
Myelinization is thus a major aspect of the workings of neural circuits.

T. Paus et al. (2000) report a computational analysis of structural magnetic resonance
images (see note below) obtained in 111 living children and adolescents. The authors
report the analysis reveals age-related increases in white-matter density in fiber
tracts constituting apparent corticospinal and frontotemporal pathways. The maturation
of the corticospinal tract was bilateral, but that of the frontotemporal pathway
was found predominantly in the left (speech-dominant) hemisphere. The authors
suggest these findings provide evidence for a gradual maturation, during late
childhood and adolescence, of fiber pathways presumably supporting motor and speech
functions. The authors also suggest their finding may provide guidance for further
investigations of neurodevelopmental disorders such as schizophrenia: "the abnormal
rate of myelinization during childhood or adolescence may very well underlie the
emergence of psychotic symptomatology." Finally, the authors suggest that the
demonstrated possibility of detecting subtle structural variations in white matter
in the living human brain opens up new avenues of research on normal and abnormal
cognitive development and in the evaluation of the long-term effects of various
treatment strategies.

Magnetic resonance imaging (MRI) is a technique for examining morphology (as opposed
to functional magnetic resonance imaging, or fMRI, which is a technique
for examining anatomical correlates of function). In general, MRI involves magnetic
coils producing a static magnetic field parallel to the long axis of the patient
or subject, combined with inner concentric magnetic coils producing a static magnetic
field perpendicular to the long axis. A radio-frequency coil specifically designed
for the head perturbs the static fields to generate a magnetic resonance image.
The interaction physics in this technique is that between the magnetic fields
and atomic nuclei in brain tissue. "Sliced" views can be obtained from any angle,
and the resolution is quite high and on the order of millimeters for magnetic
field strengths of 1.5 tesla.